Establishing the molecular basis of synaptic transmission underlies a molecular understanding of learning, behavior, and memory and is critical for the design and improvement of therapeutic agents that regulate synaptic function for treatment of psychological and neurological disorders. The highly regulated delivery of neurotransmitters to the synapse is mediated by vesicles that dock and fuse with the synaptic membrane, using molecular machinery common to all eukaryotic cells. The synaptic membrane protein syntaxin is essential for membrane fusion and has a central role in dictating specificity of membrane targeting through isoform-specific interactions with components of the vesicle membrane. This project seeks to understand syntaxin function in terms of three-dimensional structure. (1) The structures of syntaxin lA fragments containing the regions of interaction with syntaxin-binding partners will be determined by x-ray crystalography. (2) How do cognate sets of molecules on vesicle and target membranes interact? The interaction regions between syntaxin lA and VAMP 2 will be cocrystallized and the structure determined to directly examine the structural determinants of isoform specificity. (3) To assess how the conserved cytoplasmic machinery that drives vesicle fusion interacts with syntaxin prior to fusion, the structure of a syntaxin-alpha-SNAP complex will be determined. (4) How do interactions with regulatory proteins affect syntaxin structure and influence its interactions with its binding partners in the core fusion complex? The structure of the binary complex between syntaxin and the regulatory molecule n-sec 1 will be determined.